The present invention relates to an ultrasonic diagnostic apparatus for imaging a contrast echo from an ultrasound contrast medium to observe and quantify the bloodstream movement and the circulation movement of the blood on the internal organs parenchyma level by detecting perfusion.
Ultrasound is applied to various kinds of medical apparatuses, mainly an ultrasonic diagnostic apparatus for obtaining a tomogram of tissue of soft parts of a living body by means of the ultrasonic pulse reflection. The ultrasonic diagnostic apparatus obtains and displays a tomogram of tissue by means of a non-invasive examination method. The apparatus has the following advantages which cannot be obtained by the other kinds of diagnostic apparatus, such as X-ray diagnostic apparatus, X-ray computed tomographs, magnetic resonance imaging apparatuses (MRIs) and nuclear medical diagnostic apparatuses. The ultrasonic diagnostic apparatus can accomplish real-time display.
It is compact and inexpensive. It is improved in safety, since the subject is not exposed to X-ray or the like. Further, it can perform bloodstream imaging by the Doppler ultrasonography. For this reason, ultrasonic diagnosis is widely employed in the field of the heart, the abdominal organs, the mammary gland, the urinary organs, and obstetrics and gynecology. In particular, heartbeat or movement of a fetus can be display in real time by a simple operation of only applying an ultrasound probe to the body surface. In addition, since the apparatus is safe, examination can be repeated many times. Moreover, the apparatus can be easily moved to a bedside, so that examination can be carried out there.
(Contrast Medium)
To examine the heart or the abdominal organs using the ultrasonic diagnostic apparatus described above, injection of an ultrasound contrast medium through a vein and evaluation of the bloodstream movement by observing spread of the contrast medium have been put to practice. Since the intravenous injection of a contrast medium is less invasive than arteriographic echo using a catheter, it is considered that the diagnosis by means of the above method of evaluating bloodstream movement will be in common use.
Micro-bubbles, serving as reflectors having high reflectivity, are generally used as a contrast medium. The contrast effect of micro-bubbles is controllable, depending on the amount of injection and the concentration of the bubbles. However, since they are broken when irradiated with ultrasound, the contrast effect is maintained only for a short period of time.
Considering the site of a subject in a clinical situation, a contrast medium is supplied by the bloodstream continuously to the region of concern. Therefore, even if the micro-bubbles in the region of interest are collapsed by one irradiation of ultrasound, if new micro-bubbles are supplied to the region of interest before the next transmission of ultrasound, the contrast effect can be maintained. However, since ultrasound is generally transmitted several thousands of times a second, and the rate of the bloodstream is low in the internal organs parenchyma or a comparatively thin vessel, it is expected that there is no time for a fresh contrast medium to flow in and the contrast effect is reduced instantaneously.
The most basic diagnosis using a contrast medium is performed by detecting whether there is a contrast effect (increase in brightness) obtained by the contrast medium, thereby detecting whether there is a bloodstream in the diagnosis site. An improved diagnosis is carried out by observing a change with time of spatial distribution of a contrast medium in the diagnosis site in view of a spread of a change in brightness and an increase in brightness. It is also carried out by obtaining a time between the injection of the contrast medium and the arrival thereof to the region of interest (ROI), a change in time (TIC: Time Intensity Curve) of echo brightness in the ROI caused by the contrast medium, and the maximum brightness.
(Harmonic Imaging)
The contrast echo method using the contrast medium described above achieves more effective diagnosis by means of harmonic imaging. The harmonic imaging is a method for imaging a harmonic component generating from non-linear behavior caused by ultrasonic excitation, and separating it from a basic frequency component. In the living internal organs, since ultrasound has less tendency to exhibit non-linear behavior, the contrast medium can be observed at a satisfactory contrast ratio.
(Flash Echo Imaging)
As regards a method for positively utilizing the aforementioned phenomenon in which micro-bubbles are collapsed by ultrasonic irradiation, improvement of the brightness increasing effect is reported in "Flash Echo Imaging" (Reference document: 6795 Study of Flash Echo Imaging (1), Naohisa Kamiyama et al., 67th Meeting for Presenting Research Papers of Japanese Society of Ultrasonic Medicine and "Transient Response Imaging". In the method described in these documents, in principle, the conventional successive scan for scanning several frames per second is replaced by intermittent transmission for transmitting one frame in several seconds, thereby providing a time in which micro-bubbles are allowed to flow into the region of interest. When the micro-bubbles collect, ultrasound of a high sound pressure is transmitted and received, so that an echo signal of a high contrast can be obtained.
(Continuous Infusion)
A contrast medium can be administered into the body by various methods. Representative methods are: a bolus administration method for administering a contrast medium from an injector to a subject in a relatively short period of time; and a continuous infusion method for administering it little by little in a long period of time, for example, drip injection. The former is suitable for obtaining TIC, since it is relatively easy of administration and provides increased brightness at the peak when the contrast medium reaches the region of interest. However, the period, in which the contrast medium is maintained to a satisfactory concentration, is short. The latter method must be controlled by a special-purpose device, such as a constant infusion device; however, it is advantageous in that the concentration can be maintained for a satisfactory period of time. In the case of using an improved contrast medium, which has a sufficient contrast effect even if it is diluted, the latter method is more effective.
As described above, perfusion of internal organs has been able to be imaged by means of the intravenous contrast medium and the imaging method, such as harmonic imaging or flash echo imaging. However, it is known that the imaging of perfusion is not always possible but can hardly be detected because of the following problems.
The principle of harmonic imaging is to image only a harmonic component mainly resulting from non-linear reflection by the micro-bubbles in the contrast medium. The principle is based on the assumption that the level of a harmonic component resulting from non-linear transmission in tissue of the internal organs is much lower than that of a harmonic component resulting from micro-bubbles. However, the level of the harmonic component resulting from the internal organs is different from person to person. In many subjects, the level of the harmonic component resulting from the internal organs is comparatively high. In this case, as a result, even before administering the contrast medium, the echo brightness of the internal organs may be high. Particularly, in a portion where the bloodstream is very weak, after the contrast medium is administered, it is difficult to confirm whether the echo brightness is caused by the contrast medium.
In the experience of the inventor, when diagnosing the internal organs, the echo brightness in the case of cirrhosis of the liver or fatty liver is greatly different from those in the other cases. The same applies to detection of perfusion in the cardiac muscle. The cardiac muscle tissue also provides a number of harmonic components of comparatively high level in an echo signal. In particular, since an echo signal from the posterior wall of the cardiac muscle passes the ventriculus cordis, the waveform thereof is distorted by the blood (liquid) in that portion, resulting in increase in harmonic components. For this reason, even when a contrast medium is administered, the posterior wall of the cardiac muscle tends to provide high echo brightness.